(1) Field of the Invention
This invention relates generally to X-ray generators, and more particularly to X-ray generators having rotatable anodes.
(2) Prior Art
A rotatable anode X-ray tube may comprise a disc-like anode rotatably supported in a chamber, the anode being a target, an annular portion of which defines a focal track. The focal track is usually constructed from a high atomic number material such as tungsten or molybdenum which readily emits X-rays when it is bombarded by high energy electrons. A source of electrons is arranged to direct a high energy beam or to the focal track and thereby generate directable X-rays therefrom. The focal track portion of the anode disc is generally disposed at a predetermined target angle with respect to the plane of the disc so that target area or focal spot area is inclined toward an X-ray transparent window on the wall of the chamber. Thus, the X-rays pass through the window in a beam and appear to be emanating as an angled projection from the focal spot area within the tube.
A substantial portion of the electron beam energy that strikes the focal track area is converted to heat which is manifested by a sharp increase in temperature of the target material, frequently as high as 3000.degree. C. In order to avoid pitting or otherwise damaging the focal track surface, the anode disc is rotated at high angular velocities, frequently in the order of 10,000 to 20,000 RPM for example, to move successive segments of the focal track rapidly through the focal spot area that is aligned with the electron beam. As the focal track and target disc rotate, the particular areas which are not being struck with the electrons from the cathode are given an opportunity to cool through radiant dissipation of the heat. Though some heat is dissipated through radiant energy, the heat build up in the disc is frequently greater than the amounts which are dissipated and when the electron beam continues to impinge upon the same track in subsequent rotations of the target disc, the material may become over-heated and possibly permanently damaged. Also, if the tube is allowed to over-heat, the bearings on the shaft which support the disc within the chamber can become inoperative.
Rotating anode target structures which have been disclosed in the prior art include a beveled edge disc with the cathode beam impinging upon the beveled surface which in turn generates the X-rays and reflects them through the window. Frequently, the beveled portion of the target anode is a layered bimetallic construction which readily conducts heat from the focal track area into the body of the anode. Such structures are not wholly satisfactory and do not allow maximum loads of cathode ray bombardment over a reasonably long period of time without causing over-heating.
Rotating anode X-ray tubes in which the electrons impinge upon changing surfaces are also known to the art. U.S. Pat. No. 3,836,805 to Kok discloses an anode that is carried on a rotor of a motor, the rotor being driven by a stator and including a pinion gear movable on a slide so that the electron spot changes along the axis of the anode disc as the anode and the rotor are rotated. The anode is shifted axially in response to a heat sensing device which drives the pinion gear and which thereupon changes the positioning of the focal track. With the disclosed construction, the anode can produce substantial amounts of off-focus radiation which especially occurs because the cathode rays are directed at right angles to the surface of the anode.
The U.S. Pat. No. 2,926,270 to Zunick discloses a disc shaped, bevel-type anode which is rotated and wobbled to alter the track upon which the electrons impinge. Upon continual use, this relatively heavy anode can misalign from the desired axial setting of the tube and produce off-focus radiation.
Recently issued U.S. Pat. No. 4,162,420 to Grady discloses an X-ray tube which includes a rotatable anode disc having a peripheral surface which is parallel to the axis of disc rotation, the peripheral surface comprising the focal track of the disc. Electron beams are directed against the peripheral surface and reflect away as X-rays through an appropriate window nearby. The rotative disc is axially reciprocable to permit changes in the locus of points defining the focal track along the peripheral surface.
It is an object of the present invention to provide a rotative disc anode for an X-ray tube, which anode provides for substantial variation of the focal path during disc rotation.
It is a further object of the present invention to provide a rotative disc anode for an X-ray tube which anode provides for excellent heat dissipation during electron beam bombardment thereof to prevent over-heating of the anode.